Archaeologists excavating at Lincoln Castle in England have discovered a mysterious tunnel under the courtyard. The tunnel is linked to a circular structure of unknown use and delves into the ground. The archaeological team hasn’t finished its work or discovered where the tunnel leads.

Europe has many traditions of hidden tunnels connecting buildings or going to secret caves or hideouts.

The excavation is taking place in preparation for building an elevator to allow handicapped access to the walls. The archaeologists believe the structure and tunnel could date to the early 12th century. It’s the only medieval building to have been found inside the castle’s bailey.

Lincoln Castle is a well-preserved Norman castle built in 1068 on the foundations of an earlier Roman fort. William the Conqueror, after defeating the English king at the Battle of Hastings in 1066, built this castle to control the important town of Lincoln and its surrounding area. The structure has been added to over the years but remains an excellent example of Norman architecture.